Activation, regulation and physiology of natural competence in Lactococcus lactis
Lactococcus lactis is a Gram-positive bacterium that is mainly used in dairy products and safe for consumption. Within species, strains might harbour different traits which makes one strain suitable for milk fermentation whereas the other strain is not. However, natural improvement of strain performance important point within the dairy industry. Many bacteria can perform horizontal gene transfer which encompasses natural mechanisms to take up DNA. These mechanisms include phage transduction, nanotubes, conjugation and natural competence. In this study, we examined whether L. lactis can develop natural competence. Strains that appear to have a complete set of competence genes were selected following comparative genomic analysis. A construct, comprising the gene comX encoding the master regulator of competence under control of the nisin promotor, was introduced into the L. lactis strain KF147 which produces ComX upon addition of nisin to the growth medium of L. lactis. Indeed, moderate expression of ComX allowed DNA-uptake from the environment but also efficient integration of homologous fragments into the genome. In addition, the pleiotropic stringent response regulator CodY appears to be involved in the natural competence state and high expression of ComX leads to stagnation of growth, overall stress/ stringent response, however, these cells remain metabolically active. The natural trigger for natural competence could not be established. However, our research shows that there is strong evidence that environmental factors leading to starvation or stringent response in L. lactis might possibly trigger natural competence.